Oriented polypropylene films are useful and widely accepted packaging films because of their good moisture barrier, stiffness, high strength, and optical properties. However, films of polypropylene do not, in general, exhibit good heat sealing properties which is an important consideration in packaging applications. This is because polypropylene films have very narrow sealing temperature ranges.
The sealing temperature range for a thermoplastic film can be considered as beginning at that point where upon application of heat and pressure the surfaces of the film will seal or bond together. As temperature is increased further, a point is reached where the thermoplastic melts and flows quite readily thus making it difficult to achieve a seal in a specific area thus marking the upper end of the sealing range. Also at the upper end of the sealing range the film may tend to burn and char leaving weak and unsightly seals. For propylene homopolymer film, as temperature is increased, the film does not become sealable but rather suddenly flows undesirably as its melting point is approached making it quite difficult to achieve smooth, continuous, satisfactory seals. This type of sealing performance can not be tolerated on high speed packaging equipment.
In order to get good heat sealability, it has been the practice to apply various types of coatings to the films. Coatings have conventionally been applied in separate coating operations, such as from emulsions, extrusion coating, and so forth, but such operations are costly and require additional handling of the film. Coextrusion has been used successfully to put a heat sealable coating on polypropylene and produce a film of good seal strength, but until the present invention coextruded polypropylene film laminates had a very narrow heat seal range, and were expensive to produce. Accordingly, it is a principal object of the present invention to provide a polypropylene film having a wide sealing range.
Another object of the present invention is to provide a polypropylene film having acceptable seal strength for shrink packaging applications.
Typical patents which disclose coatings for polypropylene film are U.S. Pat. No. 3,285,766 which issued on Nov. 15, 1966 to Edward Barkis et al which discloses a method of extruding a coating containing at least 65% ethylene onto a sheet of polypropylene; U.S. Pat. No. 3,671,383 which issued on June 20, 1972 to Rikita Sakata et al which discloses a biaxially oriented polypropylene film laminated to a uniaxially oriented ethylene-propylene copolymer film with at least 75% by weight polypropylene; U.S. Pat. No. 4,132,050 which issued on Jan. 2, 1979 to Roger N. Young and which discloses a film having a substrate formed of a blend of 87.5 to 60 parts of polypropylene and 12.5 to 40 parts of ethylene-propylene block copolymer with a heat sealable layer; and, U.S. Pat. No. 4,148,972 which issued on Apr. 10, 1979 to Toshi Hiro Yamane et al which discloses a polypropylene film laminate made by laminating a polypropylene layer having 1 to 8 weight percent of an ionomer with an ionomer layer.